The present invention relates to a toner for use in a recording method or image forming method, such as electrophotography, electrostatic recording, magnetic recording or toner jetting, and an image forming method using the toner. More specifically, the present invention relates to a toner for use in an image recording apparatus applicable to a copying machine, a printer, a facsimile apparatus, a plotter, etc., and an image forming method using the toner.
Hitherto, a large number of electrophotographic processes have been known, inclusive of those disclosed in U.S. Pat. Nos. 2,297,691; 3,666,363; and 4,071,361. In these processes, in general, an electrostatic latent image is formed on a photosensitive member comprising a photoconductive material by various means, then the latent image is developed with a toner, and the resultant toner image is transferred via or without via an intermediate transfer member onto a transfer(-receiving) material such as paper etc., as desired, fixed by heating, pressing, or heating and pressing, or with solvent vapor to obtain a copy or print carrying a fixed toner image. A portion of the toner remaining on the photosensitive member without being transferred is cleaned by various means, and the above mentioned steps are repeated for a subsequent cycle of image formation.
An example of ordinary full-color image forming process will now be described. A photosensitive member (electrostatic image-bearing member) in the form of a drum is uniformly charged by a primary charger and then subjected to imagewise exposure with laser light modulated by a magenta image signal obtained from an original to form an electrostatic image on the photosensitive drum, which is then developed with a magenta toner contained in a magenta developing device to form a magenta toner image. Then, the magenta toner image formed on the photosensitive drum is transferred directly or indirectly onto a transfer material under the action of a transfer charger.
The photosensitive drum after the above-mentioned developing of an electrostatic image is charge-removed by a charge-removing charger and cleaned by a cleaning means so as to be prepared for a subsequent cyan-image forming cycle including charging again by the primary charger, a cyan toner image formation and a transfer of the cyan toner image onto the transfer material carrying the magenta toner image already transferred thereto, followed further by a yellow-image forming cycle and a black image forming cycle to provide the transfer material with four-color toner images thereon. Then, the transfer material carrying the four-color toner images is subjected to fixation under application of heat and pressure, thereby forming a full-color image.
In recent years, an image-forming apparatus performing an image forming method as described above not only is used as a business copier for simply reproducing an original but also has been used as a printer, typically a laser beam printer (LBP), for computer output, and a personal copier (PC) for individual users.
In addition to such uses as representatively satisfied by a laser beam printer, the application of the basic image forming mechanism to a plain paper facsimile apparatus is also popular.
For such uses, the image forming apparatus has been required to be smaller in size and weight and satisfy higher speed, higher quality and higher reliability. Accordingly, the apparatus has been composed of simpler elements in various respects. As a result, the toner used therefor is required to show higher performances. Further, in accordance with various needs for copying and printing, a greater demand is urged for color image formation, and a higher image quality and a higher resolution are required for faithfully reproducing an original color image. There is also an increasing demand for an image forming system allowing the formation of an image sheet having images on both sides from an original sheet having images on both sides.
In order to comply with the demands for a toner used in such a color image forming process, each color toner is required to exhibit excellent meltability and color-mixing characteristic on heating under application of a pressure. For this purpose, it is preferred to use a toner having a low softening point and a melt-viscosity which sharply decreases down to a low value below a prescribed temperature (i.e., having a high degree of sharp melting characteristic). By using such a toner, it is possible to provide a color copy satisfying a broader range of color reproducibility and faithful to the original image.
However, such a color toner having a high degree of sharp meltability generally has a high affinity to a fixing roller and is liable to cause offsetting onto the fixing roller at the time of fixation.
Particularly, in the case of a fixing device for a color image forming apparatus, a plurality of toner layers including those of magenta toner, cyan toner, yellow toner and black toner, are formed on a transfer-receiving material, so that the offset is liable to be caused as a result of an increased toner layer thickness.
Hitherto, in order to prevent the attachment of a toner onto a fixing roller surface, it has been practiced to compose the roller surface of a material, such as a silicone rubber or a fluorine-containing resin, showing excellent releasability against a toner, and coat the roller surface with a film of a liquid showing a high releasability, such as silicone oil or a fluorine-containing oil, for the purpose of preventing offset and deterioration of the roller surface. However, such a measure, though very effective for preventing toner offset, requires an equipment for supplying the offset-preventing liquid and complicates the fixing device. Further, the oil application is liable to promote a peeling between layers constituting the fixing roller, thus causing a shorter life of the fixing roller.
Accordingly, based on a concept of not using such a silicone oil-supplying device but supplying an offset-preventing liquid from toner particles on heating under pressure, it has been proposed to incorporate a release agent, such as low-molecular weight polyethylene or low-molecular weight polypropylene within toner particles.
For example, the incorporation of a wax in toner particles has been disclosed in Japanese Patent Publication (JP-B) 52-3304, JP-B 52-3305 and Japanese Laid-Open Patent Application (JP-A) 57-52574.
Further, the incorporation of a wax in toner particles is also disclosed in JP-A 3-50559, JP-A 2-79860, JP-A 1-109359, JP-A 62-14166, JP-A 61-273554, JP-A 61-94062, JP-A 61-138259, JP-A 60-252361, JP-A 60-252360 and JP-A 60-217366.
Wax has been used in order to provide improved anti-offset characteristic of the toner at low or high temperatures, and also an improved fixability at low temperatures. On the other hand, the resultant toner is liable to have a lower anti-blocking property or inferior developing performance due to migration of the wax to the surface of toner particles when exposed to heat due to a temperature increase in a copying machine or due to a long term of standing of the toner.
For such problems, a great expectation has been imparted to development of a novel toner.
For complying with such an expectation, a toner obtained through a suspension polymerization process has been proposed (JP-B 36-10231). In the suspension polymerization process, a monomer composition is prepared by uniformly mixing (i.e., dissolving or dispersing) a polymerizable monomer and a colorant, and optionally a polymerization initiator, a crosslinking agent, a charge control agent, and other additives, and the monomer composition is dispersed in an aqueous medium containing a dispersion stabilizer under the action of an appropriate stirrer, and subjected to polymerization, thereby providing toner particles having a desired particle size.
In the suspension polymerization process, the monomer composition is dispersed into liquid droplets in a dispersion medium, such as water, having a large polarity. Accordingly, a component having a polar group contained in the monomer composition is concentrated at the surface of the droplets, i.e., the boundary with the aqueous phase, and non-polar components are predominantly present at the inner part, thus providing a so-called core/shell structure. Thus, by enclosing a wax component as a release agent, a polymerization process toner can satisfy, in combination, low-temperature fixability, and anti-blocking property, durability and anti-high-temperature offset property, which are generally contradictory with each other. Further, it is also possible to prevent high-temperature offset without applying a release agent, such as oil, onto the fixing roller.
JP-A 6-194877 has disclosed toner particles having a so-called sea-island-sea texture wherein a crystalline (meth)acrylate polymer is dispersed in a matrix of binder resin as a plurality of domains each in turn containing a plurality of domains of the binder resin. By using behenyl (meth)acrylate having a relatively high melt-viscosity as an anti-high-temperature offset agent, it is possible to obtain a toner having excellent anti-high temperature offset property. However, as the crystalline behenyl (meth)acrylate has an excessively high melt-viscosity, the resultant toner is liable to show inferior low-temperature-fixability, thus requiring a further improvement in this respect. Further, as the anti-high-temperature offset agent is crystalline, it provides a fixed toner image exhibiting poor optical transmittance when formed on an OHP film, so that the application thereof to a full color toner is difficult.
Further, even in a toner having such a sea-island-sea texture, the texture is liable to collapse when the toner contains much residual monomer or is left standing for a long term to have the resin in the wax become missible with the wax, whereby the effect attributable to the sea-island-sea texture cannot be sufficiently achieved to cause a lowering in mechanical strength of the toner particles.
Incidentally, each toner particle contains a colorant of various pigment or dye as an indispensable component, and many of such colorants are somewhat hygroscopic, thus being liable to result in a problem regarding environmental stability. As an improvement to the problem, JP-A 63-19663 has disclosed a spherical toner with a suppressed amount of carbon black exposed to toner particle surfaces; and JP-A 5-289396 has disclosed full-color toner particles each containing one of yellow, magenta and cyan colorants while suppressing the exposure of the colorants to the toner particle surfaces, by finely dispersing resin domains containing such colorants dispersed therein in a thermoplastic matrix resin in the presence of a dispersion aid. According to the teaching of these references, it is possible to obtain a toner exhibiting stable chargeability regardless of environmental humidity by suppressing the exposure of colorant to toner particle surfaces. However, the toner of JP-A 63-19663 is liable to provide images insufficient in blackness, and the toner of JP-A 5-289396 is liable to have insufficient low-temperature fixability.
On the other hand, JP-A 4-73662 has disclosed toner particles having an outer shell of insulating resin layer formed by a mechano-chemical reaction and containing a high dielectric electroconductivity-imparting substance, such as carbon black, enclosed within the insulating resin layer. However, the toner of this reference has left room for improvement regarding blackness and gloss.
Hitherto, in full color copying machines, there has been frequently included a full-color image forming system wherein four photosensitive members and a transfer belt are included, and cyan, magenta, yellow and black toner images formed on the respective photosensitive member by developing electrostatic latent images thereon with respective color toners are successively transferred onto a recording sheet carried on the transfer belt and conveyed to positions disposed between the respective photosensitive members and the transfer belt along a straight pass, thereby forming a full-color image; or a system wherein a recording sheet is wound by an electrostatic force or a mechanical action as by a gripper about the surface of a transfer drum disposed opposite to a photosensitive member and developing and transfer steps are repeated in four cycles, to form a full color image.
Further, in recent years, as copying or recording sheets for full-color recording, there has been an increasing demand to use a variety of materials inclusive of a thick paper or card, and a small-size paper such as a post card, in addition to conventionally used plain paper or overhead projector (OHP) films. In the above-mentioned system using four photosensitive members, the recording sheet is transferred along a straight pass, so that the system is applicable to a broad range of recording sheet materials. In the system, however, plural toner images required to be superposed in registration with each other on the recording sheet at prescribed positions and even a slight deviation in registration leads to a failure in production in high-quality images at a good reproducibility, thus requiring a complicated conveying mechanism, resulting in a lowering in reliability and an increase in number of parts. On the other hand, in the system wherein the recording sheet attached onto and wound about the transfer member, a thick paper having a large basis weight when used as the recording sheet is liable to cause a failure in attachment at a trailing end thereof due to its stiffness thus being liable to results in image defects due to transfer failure. Such image defects are liable to occur also on small-size papers.
A full-color imaging apparatus using a drum-shaped intermediate transfer member is also known as disclosed in U.S. Pat. No. 5,187,526 and JP-A 4-16426. The U.S. Patent describes that high-quality images can be formed by using an intermediate transfer roller having a polyurethane-based surface layer having a volumetric resistivity of below 109 ohm.cm in combination with a transfer roller with a similar surface layer but having a volume resistivity of at least 1010 ohm.cm. However, in order to supply a sufficient transfer charge to a toner image to be transferred in such a system, a high output electric field is required so that the surface layer composed of polyurethane with an electroconductivity-imparting material dispersed therein is liable to cause local breakdown, where noticeable image disorder is generated when forming a halftone image of a small toner coverage. Moreover, such a high-voltage application is liable to result in a transfer failure due to transfer current leakage caused by a lowering in resistivity of recording sheet when used in a high-humidity environment of a relative humidity exceeding 60% RH and can also result in a transfer failure due to a non-uniform resistivity of the recording sheet even in a low-humidity environment of a relative humidity below 40% RH in some cases.
JP-A 59-15739 and JP-A 59-5046 disclose a relation between a system using an intermediate transfer member and a toner used therein. However, these references merely disclose an effective transfer of a toner of 10 xcexcm or smaller by using an adhesive intermediate transfer member, and a toner image is once transferred from a photosensitive member to the intermediate transfer member and then transferred from the intermediate transfer member to a recording sheet, so that the transfer efficiency has to be increased compared with the above-mentioned conventional systems. Particularly, in the case of a full-color copying machine compared with a monochromatic copying machine using a single black toner, the amount of toners held on the photosensitive member is increased, so that it is difficult to increase the transfer efficiency simply by using a conventional toner. Further, in the case of using a conventional toner, due to a shearing force or rubbing force acting between the photosensitive member or the intermediate transfer member and a cleaning member, and/or between the photosensitive member and the intermediate transfer member, the melt-sticking or filming of the toner onto the surface of the photosensitive member or the intermediate transfer member is liable to occur to cause a lowering in transfer efficiency and/or a failure in uniform transfer of four-color toner images in full-color image formation leading color irregularity or a problem in color balance, so that it is difficult to stably output high-image quality full-color images.
Further, respective color toners charged in an ordinary full-color copying machine are required to cause sufficient color mixing with each other in the fixing step to provide good color reproducibility and good transparency for OHP images, so that such color toners may generally preferably comprise a sharp-melting resin of a lower molecular weight than a black toner. An ordinary black toner contains a release agent having a relatively high crystallinity, as represented by polyethylene wax or polypropylene wax, in order to provide a good anti-high-temperature offset property at the time of fixation. In the case of full-color toners, however, such a crystalline release agent results in an OHP toner image having a remarkably lower transparency. For this reason, such a release agent is not added as an ordinary color toner component, but silicone oil, etc., is applied onto a heat-fixing roller to improve the anti-high-temperature offset property. However, a record sheet carrying the thus-fixed toner image retains excessive silicone oil, etc., attached thereto, so that a user can feel unpleasant in use thereof. As described above, a full-color image forming system using an intermediate transfer member and having many transfer positions has left problems to be solved at present. The above-mentioned references JP-A 59-15739 and JP-A 59-5046 have not proposed solutions to these problems regarding the toner and intermediate transfer member.
An object of the present invention is to provide a toner having good low-temperature fixability and storage stability and also good continuous image forming characteristics, and an image forming method using such a toner.
Another object of the present invention is to provide a toner capable of providing with good hue and proper gloss, and an image forming method using such a toner.
According to the present invention, there is provided a toner, comprising: toner particles each containing a binder resin, a colorant and a wax; wherein
each toner particle has such a microtexture as to provide a cross section as observed through a transmission electron microscope (TEM) exhibiting a matrix of the binder resin, a particle of the wax enclosed with the matrix, and a resin dispersed in a particulate form in the wax particle, and
the toner particles have a residual monomer content of at most 500 ppm by weight of the toner particles.
According to another aspect of the present invention, there is also provided a toner, comprising: toner particles each containing a binder resin, a colorant and a wax; wherein
each toner particle has such a microtexture as to provide a cross section exhibiting a matrix of the binder resin, and a particle of the wax enclosed within the matrix, and the colorant is dispersed to provide a projection area (B) in the binder resin and a projection area (W) in the wax giving a ratio B/W of 0/100-80/20, respectively as observed through a transmission microscope (TEM).
According to still another aspect of the present invention, there is provided an image forming method, comprising:
a charging step of charging an image-bearing member,
an electrostatic image forming step of forming an electrostatic image on the charged image-bearing member;
a developing step of developing the electrostatic image with either one of the above-mentioned toners carried on a developer-carrying member to form a toner image on the image bearing member,
a transfer step of transferring the toner image on the image-bearing member to an intermediate transfer member onto a recording material, and
a fixing step of heat-fixing the toner image on the recording material.
According to a further aspect of the present invention, there is provided an image forming method, comprising:
a charging step of charging an image-bearing member,
an electrostatic image forming step of forming an electrostatic image on the charged image-bearing member;
a developing step of developing the electrostatic image with either one of the above-mentioned toners carried on a developer-carrying member to form a toner image on the image bearing member,
a first transfer step of transferring the toner image on the image-bearing member to an intermediate transfer member,
a second transfer step of transferring the toner image on the intermediate transfer member onto a recording material, and
a fixing step of heat-fixing the toner image on the recording material.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.